This invention relates to optical amplifiers, and particularly to optical amplifiers for providing broadband amplification.
Various rare-earth doped optical amplifiers are known, such as Erbium or Erbium-Ytterbium doped fibers, and these are used to compensate for the fiber link and splitting losses within optical communications system. Pump light from a pump source is used to excite the dopant atoms in the fiber. Emission of energy from the excited atoms is stimulated by the incident signal, and this emission results in amplification of the signal.
The gain of rare-earth doped fibers as a function of the wavelength of the signal to be amplified typically includes a magnum gain in the form of a plateau, which provides the most useful operating region of the amplifier. It is desirable to provide a flat gain over the operating wavelength range, and various gain flattening filters are used for this purpose. However, the width of the plateau in the gain profile provides a limitation to the range of wavelengths for which the amplifier can be used,
Different rare-earth dopants will provide different gain profiles, and amplifier arrangements have been proposed which place different types of amplifier in parallel, so that amplification across a broader wavelength range can be achieved. Wavelength-dependent optical splitting devices are required to implement this type of amplifier. Parallel connection of the amplifiers is required because signals outside the useful operating wavelength range of each amplifier need to bypass the amplifier, as the amplifiers are attenuating at wavelengths far enough outside their operating wavelength. The different amplifiers will have different pump requirements, and the need for different pump wavelength has in the past required multiple pump sources.
The characteristics of practical amplifiers have lead to the definition of three wavelength bans: the S band (1450 nm-1520 nm); the C band (1527 nm-1563 nm);and the L band (1570 nm-1603 nm). A 7 nm guard band is provided between the bands.
A Raman amplifier is another known amplifier configuration. This amplifier uses conventional fiber, which is may be co- or counter-purr to provide amplification over a wavelength range which is a function of the pump wavelength. The Ramnan amplifier relies upon forward or backward stimulated Raman scattering. Typically, the pump source is selected to have a wavelength of around 100 nm below the wavelength over which amplification is required. This type of amplifier has die advantage that it does not attenuate signals outside the wavelength range over which amplification takes place. However, high power pump sources are required, and it may be difficult in practice to implement pump sources of the required pump wavelength and power. The use of multiple Raman amplifiers at different wavelengths has also beau proposed, but this has in the past required multiple high power pump sources.
According to a first aspect of the invention, there is provided an optical amplifier arrangement comprising at least first and second pumped amplifiers, having different pump wavelength requirements, wherein a single pump source provides the pump light for the first and second amplifiers, at least one wavelength converting arrangement being provided for converting the pump source wavelength.
This arrangement enables a single pump source to provide multiple pump signals for different amplifiers. The amplifiers may comprise any combination of amplifiers with different pump requirements, for example the combination of a rare-earth doped amplifier and a Raman amplifier, or the combination of multiple Raman amplifiers.
The first amplifier may comprise a rare-earth doped amplifier, for example Thulium, and the second amplifier may comprise a Raman amplifier. Preferably, a splitter is provided for splitting the output of the pump source into two branches, one branch being provided as pump source to the first amplifier, and the second branch being provided to the wavelength converting arrangement, the output of the wavelength converting arrangement being provided as pump source to the second amplifier.
According to a second aspect of the invention, there is provided an optical amplifier comprising a pumped Thulium doped fiber amplifier and a Raman amplifier provided in series, the Thulium doped fiber amplifier having a maximum gain at a first wavelength, the Raman amplifier being pumped at a wavelength such that the Raman amplifier has a maximum gain adjacent the first wavelength, the amplifier thereby providing useful gain over a wavelength range which is broader than the range over which the Thulium amplifier provides useful gain.
This xe2x80x9cuseful gainxe2x80x9d may be defined as the wavelength range over which the gain remains within a predetermined level compared to the maximum gain, for example the wavelength range over which the gain is within 3 dB of the gain.
The two amplifiers can be placed in series, as the Raman amplifier does not attenuate the gain provided by the Thulium amplifier, and this avoids the need for wavelength-selective splitters. The amplifier has increased bandwidth, thereby allowing it to be used in higher capacity optical communications systems. The Raman amplifier preferably is pumped to have a substantially flat gain spectrum above the wavelength range where there is useful gain provided by the Thulium amplifier. The Ramnan amplifier may then be pumped by a Raman laser providing a 1413 nm output. In this case, the Thulium amplifier operates in the S band, but the wavelength range is extended to the C band by the Raman amplifier.
Preferably, a single pump source provides the pump light for the Thulium doped amplifier and for the Raman amplifier. A wavelength converting arrangement is then provided for converting the pump source wavelength.
This reduces the number of components required to provide the pump signals. A splitter is preferably provided for dividing the pump source output into first and second outputs, the first output being provided as pump source for the Thulium doped amplifier and the second output being provided to the wavelength converting arrangement for supply to the Raman amplifier. The pump source may be a 1061 nm laser pump source, and the wavelength converting arrangement (a set of reflection Bragg gratings and a length of dispersion shifted fiber) increases the wavelength to 1413 nm.
The amplifier of the invention can be used in a wavelength division multiplex (WDM) optical communications system comprising a transmitter for generating signal radiation of wavelength in an operating wavelength range, a receiver for receiving for detecting the signal radiation, and an optical fiber link between the transmitter and the receiver. One or more of the optical amplifiers are provided in the link.
The invention also provides a method of designing a broadband optical amplifier.